class Solution {
    public int[][] matrixBlockSum(int[][] mat, int k) {
        int m = mat.length,n = mat[0].length;
        int[][] dp = new int[m + 1][n + 1];
        for(int i = 1;i <= m;i++)
            for(int j = 1;j <= n;j++)
                dp[i][j] = dp[i][j - 1] + dp[i - 1][j] - dp[i - 1][j - 1] + mat[i - 1][j - 1];

        int[][] ret = new int[m][n];
        for(int i = 0;i < m;i++)
            for(int j = 0;j < n;j++) {
                int x1 = Math.max(0,i - k) + 1,y1 = Math.max(0,j - k) + 1;
                int x2 = Math.min(m - 1,i + k) + 1,y2 = Math.min(n - 1,j + k) + 1;
                ret[i][j] = dp[x2][y2] - dp[x1 - 1][y2] - dp[x2][y1 - 1] + dp[x1 - 1][y1 - 1];
            }
        return ret;
    }
}